The combustion process of homogeneous charge compression ignition (HCCI) engines depends strongly on factors such as cylinder charge composition, temperature, and pressure at the intake valve closing. Hence, the control inputs to the engine, for example, fuel injection mass and timing and intake/exhaust valve profile, must be carefully coordinated to ensure robust auto-ignition combustion. Generally, for best fuel economy, an HCCI engine operates un-throttled and with a lean air-fuel mixture. Further, in an HCCI engine using an exhaust recompression valve strategy, the cylinder charge temperature is controlled by trapping different amount of the hot residual gas from the previous cycle by advancing the exhaust valve close timing from nominal. The opening timing of the intake valve is retarded from nominal to a later time preferably symmetrical to the exhaust valve closing timing about top-dead-center (TDC) intake. Both the cylinder charge composition and temperature are strongly affected by the exhaust valve closing timing. In particular, more hot residual gas from the previous cycle can be retained with earlier closing of the exhaust valve which leaves less room for the incoming fresh air mass. The net effects are higher cylinder charge temperature and lower cylinder oxygen concentration. The negative valve overlap (NVO), defined as the crank-angle period where both intake and exhaust valves are simultaneously closed around TDC intake, is indicative of the trapped amount of hot residuals.
Robust HCCI combustion has been demonstrated using a variable valve actuation system such as a fully flexible valve actuation (FFVA) system (e.g. electrically variable, hydraulically variable or electro-hydraulically variable valves) or a simplified mechanical two-step valve lift system with a dual cam phasing system. In particular, optimal combustion phasing can be maintained by adjusting both intake and exhaust valve profiles in conjunction with engine control inputs such as injection mass and timing, spark timing, throttle and EGR valve positions. Furthermore, air-fuel ratio control is critical for maintaining robust HCCI combustion especially during transients.
In conventional gasoline spark-ignition engines, airflow is controlled by the throttle, and the fuel is metered proportional to the measured mass airflow at the throttle body using a MAF sensor. The noise level (i.e. high frequency components) of the MAF signal is low as long as the intake manifold absolute pressure (MAP) is far below the ambient pressure (i.e. throttled engine operation). However, during minimally throttled operation, noise levels can be substantial due to significant coupling of intake dynamics of the cylinders with the intake manifold and MAF sensor. During HCCI engine operations, the throttle is usually kept wide-open to minimize pumping losses, and the airflow is controlled by the exhaust and intake valve profiles (i.e. combinations of lift, duration and phase). Therefore, engines operating in an HCCI mode are also affected by MAF signals which can be substantially noisy. Similarly, in diesel engines, which operate without air throttling, MAF signals can similarly be substantially noisy. Although the high-frequency components in the MAF measurement can be reduced using a low pass filter, a filtered signal produces an undesirable time delay in the MAF measurement. Adapting fuel injection command using a filtered, and hence time delayed, MAF measurement can cause significant air-fuel ratio deviations during engine transient operations resulting in undesirable combustion results including, for example, partial burn, misfires, excessive emissions, combustion phase shifts, etc.